Alloys for intermediate temperature applications, methods for maufacturing thereof and articles comprising the same

ABSTRACT

Disclosed herein is a composition comprising iron; about 18 to about 30 wt % chromium; up to about 7 wt % tungsten; up to about 1.5 wt % manganese; up to about 1 wt % aluminum; about 0.02 to about 0.1 wt % of a rare earth metal and/or yttrium; wherein the weight percents are based on the total weight of the composition. Disclosed herein too is a method comprising melting together a composition comprising iron; about 18 to about 30 wt % chromium; up to about 7 wt % tungsten; up to about 1.5 wt % manganese; up to about 1 wt % aluminum; about 0.02 to about 0.1 wt % of a rare earth metal and/or yttrium; wherein the weight percents are based on the total weight of the composition; casting the composition; and rolling the composition.

BACKGROUND

This disclosure is related to ferritic stainless steels for hightemperature applications, methods for manufacturing thereof and articlescomprising the same.

Solid oxide fuel cells (SOFCs) are devices that produce energy, usuallyelectricity, from a variety of fuels using an electrochemical reaction.Oxygen transfer through the electrolyte, which improves the efficiencyof energy conversion, is greatly accelerated at temperatures above 700°C. The overall fuel to electricity conversion efficiency in SOFCs can beas high as 90% and is not limited by classical thermodynamics for heatengines (Carnot cycle). Due to their high exhaust gas temperature, SOFCshave the ability to cogenerate heat and electricity. Hybrid powergeneration systems integrating the SOFCs and turbines can have very highoverall system efficiencies.

SOFCs may be tubular or planar in assembly. The key components of anSOFC are an anode, a cathode, an electrolyte, interconnects, a manifoldand seals. The cathode is largely exposed to a hot, oxidant environment,and is generally called the air or oxygen electrode. The temperature ofthe cathode feed gas is usually about 400° C. or higher. Similarly, theanode is exposed to the fuel and is called the fuel electrode. Theinterconnects interface with the anode on the fuel side and with thecathode on the air side and are usually made using oxidation resistant,heat resistant materials such as lanthanum chromite, lanthanum strontiumchromite, ferritic stainless steels and chromium base alloys.

Highly oxidizing conditions prevail at the cathode at temperatures ofgreater than or equal to about 850° C. and high oxygen partialpressures. These, along with humidity and atmospheric moisture mayoxidize chromium present in interconnects to chromium oxides orhydroxide or oxyhydroxide that grow as cathode scales and can vaporizeto poison or deactivate the cathode. Cathode scales may grow to athickness of tens of microns after exposure for thousands of hours inthe SOFC environment in an intermediate temperature range of about 800°C. Chromium hydroxide and oxyhydroxide are particularly volatile and maydegrade the cathode. To enhance life expectancy and operationalefficiency of the SOFC cathode it is desirable to reduce or eliminatecathode degradation.

Current methods for minimizing cathode degradation in SOFCs are notadequately developed and limit the useful operating life of the SOFCs.The problem may be reduced or eliminated by frequent maintenance orcathode scale removal. This may result in cell stoppage and induce asignificant energy penalty associated with the power generation cycle.

Alternatively, non-chromium containing alloys and ceramic materials withnon-volatile chromium have been employed in interconnects. However,these materials are expensive, brittle, weak under tensile forces, orhave high resistive losses making them unsuitable for interconnectapplications. Many SOFC stacks employ interconnects and components madefrom alloys containing chromium and few suitable replacement materialsare available. The problem of high cathode degradation rates has notbeen solved.

It is therefore desirable to use ferritic stainless steels that canfacilitate a reduction in the cathode degradation rates in SOFC's thatoperate at temperatures of about 800° C.

SUMMARY

Disclosed herein is a composition comprising iron; about 18 to about 30wt % chromium; up to about 7 wt % tungsten; up to about 1.5 wt %manganese; up to about 1 wt % aluminum; about 0.02 to about 0.1 wt % ofa rare earth metal and/or yttrium; wherein the weight percents are basedon the total weight of the composition.

Disclosed herein too is a method comprising melting together acomposition comprising iron; about 18 to about 30 wt % chromium; up toabout 7 wt % tungsten; up to about 1.5 wt % manganese; up to about 1 wt% aluminum; about 0.02 to about 0.1 wt % of a rare earth metal and/oryttrium; wherein the weight percents are based on the total weight ofthe composition; casting the composition; and rolling the composition.

Disclosed herein too are articles manufactured from the composition.

DETAILED DESCRIPTION OF FIGURES

With reference now to the figures, wherein like elements are numberedalike:

FIG. 1 is a schematic depicting one exemplary embodiment of a solidoxide fuel cell (SOFC);

FIG. 2 is a schematic depicting the sandwich that is used for the ASRmeasurements;

FIG. 3 is a depiction of the test set-up for measuring the ASR of theferritic stainless steels; and

FIG. 4 depicts the electrical set-up for the platinum foils that is usedfor determining the ASR of the ferritic stainless steels.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that terms such as “top”, “bottom”,“outward”, “inward”, and the like are words of convenience and are notto be construed as limiting terms. It is to be noted that the terms“first,” “second,” and the like as used herein do not denote any order,quantity, or importance, but rather are used to distinguish one elementfrom another. The terms “a” and “an” do not denote a limitation ofquantity, but rather denote the presence of at least one of thereferenced item. The modifier “about” used in connection with a quantityis inclusive of the stated value and has the meaning dictated by thecontext (e.g., includes the degree of error associated with measurementof the particular quantity).

Disclosed herein are ferritic stainless steels that reduce oxidation andimprove chemical compatibility of the metal interconnect in solid oxidefuel cells (SOFCs) and other high temperature applications. The ferriticstainless steels can be advantageously used as interconnects in a SOFCenvironment while reducing degradation due to corrosion. The ferriticstainless steels display a low oxide growth rate, can be advantageouslyused for coefficient of thermal expansion (CTE) matching and have a lowtotal area specific resistivity (ASR) of about 5 to about 40milliohm-square centimeter (measured at 750° C.) when subjected tooxidation at about 750° C. for about 1,500 hours. The ferritic stainlesssteels advantageously comprise chromium, aluminum, tungsten, manganese,rare earth elements and/or yttrium, with the balance being iron.

With reference now to the FIG. 1, an exemplary fuel cell system 200comprises a fuel cell 30 having an anode 40, an electrolyte 60, acathode 80, an interconnect 100 and a seal 105. The cathode 80 and theinterconnect 100 are in intimate electrical communication via contact90. A fuel cell stack is obtained by repeated stacking of repeating unit180 that comprises an anode 40, electrolyte 60, cathode 80,cathode-interconnect contact 90 and interconnect 100. The fuel cell isencased between the end plates 120

As can be seen from the FIG. 1, the interconnect, connects one cell toanother electrically when multiple SOFCs are used in a stack to generateelectricity. Interconnects also serve as separators for the anode andcathode gases in addition to providing mechanical stability to the SOFCstack. Since electrical connectivity of SOFCs is the function ofinterconnects, the electrical conductivity of the materials in theinterconnect has to be high and should stay high at the operatingtemperature under the cell conditions for the entire life of the SOFC.Further, the interconnect is in physical communication with the othercomponents of the cell such as the cathode and anode. Seals are used tomake the fuel cell gas-tight to avoid the intermixing of fuel andoxidant gases and the interconnects can be in physical communicationwith the seals too. Thus, it is desirable for the interconnects to bechemically inert and to have matching coefficient of thermal expansionwith the other cell components. Even if there is reaction between theinterconnect and the electrodes, the reaction product should be a goodelectrical conductor.

In one embodiment, the ferritic stainless steel used in the interconnectcomprises chromium in an amount of greater than or equal to about 18weight percent (wt %), based on the weight of the ferritic stainlesssteel. In another embodiment, the ferritic stainless steel compriseschromium in an amount of about 18 wt % to about 30 wt %, based on theweight of the ferritic stainless steel. In yet another embodiment, theferritic stainless steel comprises chromium in an amount of about 20 wt% to about 29 wt %, based on the weight of the ferritic stainless steel.In yet another embodiment, the ferritic stainless steel compriseschromium in an amount of about 21 wt % to about 28 wt %, based on theweight of the ferritic stainless steel. An exemplary amount of chromiumis about 20 to about 25 wt %, based on the weight of the ferriticstainless steel. If less than 18 wt % of chromium is added, then acontinuous protective layer of chromium oxide may not be formed. Thisprotective layer of chromium oxide minimizes the rate of degradation ofthe ferritic stainless steel. If the chromium is added in amounts ofgreater than or equal to about 30 wt %, then the ASR will increase.There is also a risk of increased volatilization if chromium is added inamounts of greater than or equal to about 30 wt %, based on the weightof the ferritic stainless steel.

The aluminum can be present in amounts of up to about 1 wt %, based onthe weight of the ferritic stainless steel. In one embodiment, thealuminum can be present in amounts of about 0.5 to about 0.9 wt %, basedon the weight of the ferritic stainless steel. In another embodiment,the aluminum can be present in amounts of about 0.55 to about 0.85 wt %,based on the weight of the ferritic stainless steel. In yet anotherembodiment, the aluminum can be present in amounts of about 0.5 to about0.80 wt %, based on the weight of the ferritic stainless steel. Anexemplary amount of aluminum is about 0.75 wt %, based on the weight ofthe ferritic stainless steel. If aluminum is added in amounts of greaterthan or equal to about 1.0 wt %, then too much alumina may be formed inthe ferritic stainless steel thereby increasing the surface resistance.

Tungsten facilitates a reduction in the coefficient of thermal expansion(CTE) of the ferritic stainless steel. The amount of tungsten can bevaried to facilitate CTE matching between the interconnect and thosecomponents of the SOFC that it is physical communication with. Thetungsten can be present in amounts of up to about 7 wt %, based on theweight of the ferritic stainless steel. In one embodiment, the tungstencan be present in amounts of about 5 to about 6.8 wt %, based on theweight of the ferritic stainless steel. In another embodiment, thetungsten can be present in amounts of about 5.5 to about 6.5 wt %, basedon the weight of the ferritic stainless steel. An exemplary amount oftungsten is about 5 to about 7 wt %, based on the weight of the ferriticstainless steel.

The presence of manganese in the ferritic stainless steel facilitatesthe formation of a spinel phase upon oxidation. The presence ofmanganese reduces the volatilization of the chromium-containing oxidesand/or hydroxides. The manganese can be present in amounts of up toabout 1.5 wt %, based on the weight of the ferritic stainless steel. Inone embodiment, the manganese can be present in amounts of about 0.5 toabout 1.35 wt %, based on the weight of the ferritic stainless steel. Inanother embodiment, the manganese can be present in amounts of about 0.6to about 1.25 wt %, based on the weight of the ferritic stainless steel.In yet another embodiment, the manganese can be present in amounts ofabout 0.7 to about 1.2 wt %, based on the weight of the ferriticstainless steel. An exemplary amount of manganese is about 0.75 wt %,based on the weight of the ferritic stainless steel.

The rare earth elements are effective in controlling oxidation as theyeffectively block the grain boundary diffusion of chromium. An exemplaryrare earth element is lanthanum. Other rare earth metals from thelanthanide and actinide series of rare earth metals may be added tolanthanum if desired. Examples of such rare earth metals are cerium,praseodymium, neodymium, samarium, europium, gadolinium, uranium,neptunium, plutonium, or the like, or a combination comprising at leastone of the foregoing rare earth metals.

It is generally desirable to add the rare earth metals in amounts ofabout 0.02 wt % to about 0.1 wt %, based on the total weight of theferritic stainless steel. In one embodiment, the rare earth metals canbe added in amounts of about 0.05 wt % to about 0.08 wt %, based on thetotal weight of the ferritic stainless steel. In another embodiment, therare earth metals can be added in amounts of about 0.06 wt % to about0.075 wt %, based on the total weight of the ferritic stainless steel.If the rare earth metals are added in an amount of greater than or equalto about 0.1 wt %, then the cost of processing the ferritic stainlesssteel increases.

As noted above, the ferritic stainless steels can also comprise yttriumin addition to or in lieu of the rare earth metals. In one embodiment,yttrium can be added with the rare earth metals to the ferriticstainless steels. In another embodiment, the yttrium can be used toreplace the rare earth metals in the ferritic stainless steels.

In one embodiment, the rare earth metals and the yttrium can be added inamounts of about 0.0001 wt % to about 0.1 wt %, based on the totalweight of the ferritic stainless steel. In one embodiment, the rareearth metals and the yttrium can be added in amounts of about 0.005 wt %to about 0.08 wt %, based on the total weight of the ferritic stainlesssteel. In another embodiment, the rare earth metals and the yttrium canbe added in amounts of about 0.007 wt % to about 0.06 wt %, based on thetotal weight of the ferritic stainless steel. In yet another embodiment,the rare earth metals and the yttrium can be added in amounts of about0.008 wt % to about 0.05 wt %, based on the total weight of the ferriticstainless steel.

In one embodiment, in one method of manufacturing the ferritic stainlesssteel, the iron, chromium, aluminum, tungsten, manganese, rare earthelements and/or yttrium are vacuum arc melted followed by casting,forging and rolling into the final sheet form. In another embodiment,the ferritic stainless steel can be manufactured into a desired shape byother powder metallurgy based methods including, hot pressing, hotisostatic pressing, sintering, hot vacuum compaction, or the like. Anexemplary method of manufacturing the ferritic stainless steel is byvacuum arc melting followed by casting forging and rolling into finalsheet form.

After vacuum arc melting the material is then cast into an ingot. Theingot may then be forged and rolled into final sheet form. In oneembodiment, the ingot can be hot rolled at a temperature of about 1000°C., followed by cold rolling to a thickness of less than or equal toabout 2.54 millimeters. During the process of reduction in the thicknessof the cross-sectional area, periodic annealing may be performed on theferritic stainless steels.

The ferritic stainless steels advantageously display an area specificresistivity (ASR) of about 5 to about 40 milliohm-square centimeter(mohm-cm²) when used in an alloy sandwiches that are oxidized at 750° C.for 1,500 hours and an ASR of about 20 to about 120 mohm-cm² when usedin an alloy sandwiches that are oxidized at 850° C. for 1,500 hours. Theaforementioned ASR values are measured at a test temperature of 750° C.As detailed below, the alloy sandwiches contain a layer of lanthanumstrontium manganate disposed between two ferritic stainless steelplates.

The ferritic stainless steels also advantageously display a coefficientof thermal expansion (CTE) of about 11 to about 12.75 parts per millionper degree centigrade (ppm/° C.). In one embodiment, the ferriticstainless steels display a coefficient of thermal expansion (CTE) ofabout 11.75 to about 12.50 ppm/° C. In another embodiment, the ferriticstainless steels display a coefficient of thermal expansion (CTE) ofabout 11.85 to about 12.25 ppm/° C. The ferritic stainless steelsadvantageously have a thermal expansion coefficient to match to that ofthe electrolyte material that is used in commercially available SOFC'si.e., 8% yttria stabilized zirconia (YSZ), which is about 11 ppm/° C. inthe temperature range of about 20 to about 800° C.

The present disclosure is illustrated by the following non-limitingexample.

EXAMPLE

This example was performed to determine the area specific resistivity(ASR), the coefficient of thermal expansion (CTE) and the thickness ofan oxidation layer formed on the ferritic stainless steel in a solidoxide fuel cell environment. To measure the ASR, a sandwich of an LSM(lanthanum strontium material) and the ferritic stainless steel wascreated. As shown in the FIG. 2, this Sandwich Configuration comprises alayer of LSM disposed between two ferritic stainless steel plates. Thewhole assembly shown in the FIG. 2 was oxidized at high temperatures fora certain duration of time. The temperatures chosen were 750 and 850° C.respectively and the duration of time was 1,500 hours.

In order to sandwich the LSM between the ferritic stainless steelplates, 10 wt % polyvinyl alcohol (PVA) is dissolved in hot water tomake a PVA solution. LSM paste was prepared with 30 wt % of this PVAsolution, that is 70 grams of LSM was mixed with 30 grams of PVAsolution. The LSM paste was then applied to one surface of a ferriticstainless steel plate and another ferritic stainless steel plate waspressed on it. These alloy sandwiches were then oxidized at 750° C. and850° C. respectively for 1,500 hours. These oxidation temperatures werechosen because they are similar to the operating temperature of a SOFC.

In order to measure the ASR, after oxidizing the sandwiches, the top andbottom surfaces of the sandwich were polished off to remove the oxidethat is formed on the bare surfaces of the ferritic stainless steelplates. Then the sandwich is introduced into the measuring equipmentbetween the platinum foils, as shown in FIG. 3. As can be seen in theFIG. 3, the platinum foils each having two leads are in intimate contactwith the outer surfaces of the sandwich. This is depicted clearly in theFIG. 4 where two of the leads are connected to the top platinum foil andthe other two to the bottom platinum foil. One of the leads on top andone from bottom are used for passing a constant current and the otherpair for measuring the voltage drop across the sandwich.

The advantages of this configuration are a) after polishing off theoxide on the top and bottom surfaces of the sandwich, the platinum foilsmake direct contact with the alloys and b) The total ASR measured isacross two ferritic stainless steel-LSM interfaces thereby increasingthe accuracy of measurement.

A Keithley programmable constant current source (model 2400) andKeithley Nanovoltmeter (model 2182) were used for passing the constantcurrent and measuring voltage drop across the sample, respectively. Thevoltage drop was also measured by reversing the polarity of the constantcurrent and the average of the two readings was taken. This way anythermoelectric effects that may be present because of temperaturegradients in the furnace are also annulled. The temperature wasincreased at a rate of 5 degrees centigrade per minute and the data wascollected at an interval of 20 degrees both during heating and cooling.

The compositions along with the ASR results for these compositions areshown in the Table 1 below. In addition to the ASR measurements, CTEmeasurements were also made using a Netzsch DIL 402C dilatometer havingtemperature capability from 25 to 1500° C. CTE results are also shown inthe Table 1 below.

In addition samples were oxidized to determine the oxide thickness.Ferritic stainless steel pieces were coated with LSM slurry and wereoxidized at 750 and 850° C. for 1,500 hours. The oxidized alloys weremounted edge-on to determine the oxide thickness. In order to ensureperpendicularity, a couple of metallic clips were used. The samplessupported by the clips were inserted in the plastic cylindrical mold of1 inch diameter. Low viscosity epoxy resin was prepared by mixing 3parts resin and 1 part hardener. The cylindrical molds were half filledwith the resin and kept in the vacuum desiccators. The desiccator wasevacuated using a rotary pump until the epoxy started frothing andreached to the rim of the mold. The vacuum was broken so that resin sankagain. The process described above was repeated once again. Finally, themold was filled with the resin fully. The resin was allowed to cureovernight at room temperature.

The cold mounted samples were metallographically polished. In order toprovide a leakage path for the electrical current developed duringelectron microscopy, a silver contact was provided between sample andbottom of the molded plastic. The mounted samples along with the plasticwere degassed in the oven at 105° C. for 4 to 5 hours. The degassedmounted samples were coated with gold by DC sputtering. The thickness ofthe gold layer was 150 to 200 Angstroms. The oxide thickness wasmeasured in a scanning electron microscope (SEM) at a magnification of3000 to 5000. Often EDS was used as an aid for thickness measurement,wherever the boundaries of oxides were poorly defined. Thickness wasmeasured at a minimum of 5 locations. Oxide thickness results are alsoshown in the Table 1 below. TABLE 1 Oxide Oxide Thickness Thickness (μm)(μm) (LSM ASR @ (bare) coated) CTE 750° C. (Oxidized at (Oxidized at(775-100° C.) (mohm- 750° C. for 750° C. for Sample # Composition (ppm/°C.) cm²) 1,500 hours) 1,500 hours) Sample #1 Fe—25Cr—0.75Mn—0.05(La +Y)—7W 11.78 2.4 ± 0.4 1.6 ± 0.3 Sample #2 Fe—25Cr—0.75Mn—0.05(La +Y)—1Al 12.57 12 3.3 ± 1.1 2.1 ± 0.3 Sample #3 Fe—25Cr—0.75Mn—0.1(La + Y)12.23 11 2.2 ± 0.5 1.9 ± 0.4 Sample #4 Fe—25Cr—0.75Mn—0.1(La + Y)—7W—1Al12.29 12 4.4 ± 1.1 1.9 ± 0.5

From the Table 1, it can be sent that the ferritic stainless steels haveCTE's that are about 11.75 to about 12.6 ppm/° C. These CTE valuespermit closer thermal expansion match to electrolyte materials that aresuitable for use in commercially available SOFC's.

From the Table 1, it may also be seen that the ASR for the disclosedcompositions is about 11 to about 12 mohm-cm². These values of ASRrender the ferritic stainless steels useful for solid oxide fuel cellsthat operate at temperatures of about 800 to about 850° C. The averagevalue of the oxide thickness layer for the LSM coated samples is about1.9 micrometers when oxidized at 750° C. for 1,500 hours.

Thus from the examples it may be seen that the ferritic stainless steelscan be advantageously used in interconnects and other high temperatureapplications. They can be advantageously used at temperatures of up to850° C. They display good oxidation resistance leading to increasedstability of the LSM-ferritic stainless steel interface. The ferriticstainless steel also comprises elements that permit oxidation resistanceas well as chemical compatibility with other components of a SOFC.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention.

1. A composition comprising: iron; about 18 to about 30 wt % chromium;up to about 7 wt % tungsten; up to about 1.5 wt % manganese; up to about1 wt % aluminum; about 0.02 to about 0.1 wt % of a rare earth metaland/or yttrium; wherein the weight percents are based on the totalweight of the composition.
 2. The composition of claim 1, having an areaspecific resistivity of about 5 to about 40 milliohm-square centimeterat 750° C., when oxidized in a sandwich configuration at 750° C. forabout 1,500 hours.
 3. The composition of claim 1, having an areaspecific resistivity of about 20 to about 120 milliohm-square centimeterat 750° C., when oxidized in a sandwich configuration at 850° C. forabout 1,500 hours.
 4. The composition of claim 1, having a coefficientof thermal expansion of greater than or equal to about 11.75 parts permillion per degree centigrade.
 5. The composition of claim 1, having acoefficient of thermal expansion of about 11.75 to about 12.6 parts permillion per degree centigrade.
 6. The composition of claim 1, whereinthe chromium is present in an amount of about 25 wt %, based on thetotal weight of the composition.
 7. The composition of claim 1, whereinthe rare earth metal is lanthanum.
 8. The composition of claim 1,comprising about 5 to about 7 wt % tungsten.
 9. The composition of claim1, comprising about 0.5 to about 1.5 wt % manganese.
 10. The compositionof claim 1, comprising about 0.5 to about 1 wt % aluminum.
 11. Anarticle manufactured from the composition of claim
 1. 12. A methodcomprising: melting together a composition comprising: iron; about 18 toabout 30 wt % chromium; up to about 7 wt % tungsten; up to about 1.5 wt% manganese; up to about 1 wt % aluminum; about 0.02 to about 0.1 wt %of a rare earth metal and/or yttrium; wherein the weight percents arebased on the total weight of the composition. casting the composition;and rolling the composition.
 13. The method of claim 12, furthercomprising forging the composition.
 14. An article manufactured by themethod of claim 12.